This invention relates to aqueous stabilized vinyl acetate emulsion adhesives containing acidic metal salt curing agents which are modified by the addition of selected solvents as low temperature fusion aids.
Aqueous polyvinyl acetate adhesives have a number of excellent properties such as good adhesion, rapid hardening, colorless adhesive joints, and good workability. Due to the presence of hydrophilic protective colloids which are added to stabilize the emulsions against gelling during storage, the adhesive joints are not very waterproof.
The water sensitivity caused by the presence of the protective colloids, specifically polyvinyl alcohol, can be reduced by adding acidic metal salt curing agents, as disclosed in U.S. Pat. No. 3,563,851 issued Feb. 16, 1971 to W. B. Armour et al. These salts crosslink the adhesive coating or films and thus enhance their water resistance. The acidic nature of the curing agents also serves to accelerate the curing procedure. The resulting adhesives are particularly useful for exterior applications since they retain their excellent adhesive properties and physical structure even after prolonged exposure to moisture.
The basic disadvantage of the salt-containing systems was the inability of the adhesives to film form at temperatures below 13.degree. C. (55.degree. F.) and the tendency to precure very readily (their closed assembly times were less than 10 minutes). Most manufacturing specifications require film forming at temperatures as low as -1.degree. C. (30.degree. F.) and closed assembly times of up to 30 minutes. Adding 2,4,6-tris(hydroxymethyl)phenol, commonly known as trimethylol phenol (TMP), to such systems solved this problem. The low temperature fusion was reduced and the closed assembly time was increased to the required levels. The improved performance provided by TMP was believed to be due to the high trifunctionality of the additive and the increased density of the crosslinks formed with the vinyl acetate copolymers. Improvements in the thermosetting and elastomeric properties of polyvinyl acetate adhesives containing TMP are reported in U.S. Pat. No. 3,041,301 issued June 26, 1962 to W. B. Armour.
Several years ago the manufacture of TMP was discontinued and numerous chemicals were evaluated, without considerable success, for use as low temperature fusion aids. Of these, water soluble phenolics containing low levels of trifunctionality were the best. The phenolics would generally crosslink if the levels of functional groups in the vinyl acetate copolymer were adequate. However, if crosslinking did not occur, their presence had an undesirable plasticizing effect which reduced the water resistance of the adhesive bonds, particularly the boiling water resistance. This problem could only be overcome by making the system more unstable and then the storage stability of the system was poor. The performance of the adhesive was also inconsistent. A further disadvantage was the presence of free formaldehyde in the phenolics.
Hence, the search began for an improved low temperature fusion aid for stabilized vinyl acetate adhesive systems containing acidic metal salt curing agents.